1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a hydraulically operated power transmitting system for automotive vehicles having a continuously variable transmission.
2. Discussion of the Prior Art
A continuously variable transmission incorporated in a power transmitting system for motor vehicles is known, for example, according to U.S. Pat. No. 4,857,034 (laid-open Publication No. 52-98861 of unexamined Japanese Patent Application). The continuously variable transmission disclosed therein includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, a belt connecting the pulleys, and a pair of hydraulic actuators for changing diameters of the pair of pulleys.
Such a continuously variable transmission is controlled by a hydraulic control apparatus having a pressure regulating valve which is adapted to regulate the actuating pressure in the driven side actuator, i.e., the pressure in one of the two actuators which is provided on the driven one of the first and second shafts, so that the tension of the belt is adjusted to a permissible minimum value, depending upon the current amount of torque transmitted through the continuously variable transmission, and/or the current speed ratio of the transmission. In the conventional hydraulic control apparatus, the actuating pressure in the driven side actuator inevitably rises due to a centrifugal force when the driven side pulley rotates at a high speed, that is, when the vehicle is running at a relatively high speed. This centrifugal pressure rise in the driven side actuator results in unnecessarily increasing the tension of the transmission belt, thereby deteriorating the durability of the belt.
In view of the above problem, a hydraulic control apparatus is proposed in laid-open Publication No. 60-53258 of unexamined Japanese Patent Application. In this control apparatus, the rotating speed of the driven shaft of the continuously variable transmission is detected, and the centrifugal pressure rise in the driven side actuator due to the centrifugal force is calculated based on the detected speed of the driven shaft. The calculated centrifugal pressure rise is subtracted from a desired or optimum value of the line pressure to be applied to the driven side actuator, so that the nominal optimum line pressure is compensated for the centrifugal pressure rise. The actual line pressure to be applied to the driven side actuator is continuously regulated by a pressure regulating servo valve equipped with a linear solenoid, such that the line pressure coincides with the compensated optimum value.
However, the proposed hydraulic control apparatus is disadvantageous in terms of the cost of manufacture due to the use of the relatively expensive pressure regulating valve equipped with the linear solenoid.
In a continuously variable transmission as disclosed in laid-open Publication No. 62-196445 of unexamined Japanese Patent Application, a first line pressure and a second line pressure lower than the first line pressure are applied to one and the other of the pair of hydraulic actuators, respectively, so that the speed ratio of the transmission is controlled. At the same time, the tension of the belt is adjusted to the required minimum value, by regulating the second line pressure based on a suitable parameter such as the detected speed ratio or throttle opening angle. To this end, a hydraulically pilot-operated second pressure regulating valve is used for regulating the second line pressure, in addition to a first pressure regulating valve for regulating the first line pressure. At a given value of the throttle opening angle, the second line pressure as regulated by such second pressure regulating valve is continuously lowered as the actual speed ratio of the transmission increases. However, the ideal or desired curve of the second line pressure is such that the pressure decreases to a certain level with an increase in the speed ratio, but the rate of decrease in the desired or ideal second line pressure is considerably low after the pressure is lowered below that certain level. If the second pressure regulating valve is designed for obtaining the desired second line pressure when the speed ratio is relatively high, the actually regulated second line pressure tends to be unnecessarily high, causing an excessive increase of the tension of the transmission belt. This is undesirable for the durability of the belt. If the second pressure regulating valve is designed for obtaining the desired second line pressure when the speed ratio is relatively low, the regulated second line pressure tends to be lower than required to enable the transmission to operate without slipping of the belt o the pulleys.
In the continuously variable transmission disclosed in the above-identified laid-open Publication No. 62-196445, the rates of supply and discharge flows of the fluid into and out of the one and other of the two hydraulic actuators are determined by a difference between the first and second line pressures, so that the shifting response of the transmission is not affected by the speed ratio. If the first line pressure is unnecessarily high, extra power is unnecessarily consumed for driving an oil pump used as the hydraulic source of the system, resulting in a power loss of the vehicle engine. If the first line pressure is insufficient, the transmission cannot be shifted at a sufficiently high speed, or the rate of change in the speed ratio of the transmission cannot be sufficiently high. For this reason, it is desirable that the first line pressure be adjusted to a required minimum level. On the other hand, it is required that the first line pressure be set higher than the required minimum level by a suitable amount so as to accommodate a potential difference in the actual thrusts of the hydraulic actuators from one transmission to another, or accommodate a possible variation in the thrusts and operating characteristics of the actuators due to chronological change during use. In this respect, the conventional hydraulic control apparatus still suffers from power loss of the engine due to unnecessarily large power to drive the hydraulic source of the power transmitting system.
In view of the above, the assignee of the present application proposed a hydraulic control apparatus as disclosed in laid-open Publication No. 63-57952, wherein the first line pressure is regulated to be higher by a suitable amount than a higher one of the pressures within the two hydraulic actuators. This apparatus permits better regulation of the first line pressure with reduced engine power loss, irrespective of the difference in the thrusts of the hydraulic actuators of the individual transmissions (used on individual vehicles) and the chronological variation in the thrusts.
When the vehicle is stopped, however, the fluid in the hydraulic actuator on the side of the engine (first actuator) is sometimes discharged to the drain line of the system, to rapidly reduce the speed ratio of the continuously variable transmission to the lower limit. In this case, the first line pressure cannot be rapidly raised to the normal operating level when the engine is re-started while the first and second actuators are drained. This tendency causes inadequate regulation of the speed ratio of the transmission upon starting of the vehicle, and reduced drivability and driving comfort of the vehicle.